Unidirectional waveguide attenuator



United States Patent UNIDIRECTIONAL WAVEGUIDE ATTENUATOR Leonard Lewin, George Horace Brooke Thompson, and

Francois Sagnard, London, England, assignors to International Standard Electric Corporation, New York,

Filed Sept. 10, 1957, Ser. No. 683,076 Claims priority, application Great Britain Sept. 24, 1956 5 Claims. (Cl. 333-24) sioned and located Within a hollow waveguide, is applied to the ferromagnetic ceramic material, then a wave having a particular frequency incident from one direction can be passed through the waveguide with only slight attenuation, whereas the said wave when incident from the opposite direction will be substantially attenuated. The said particular frequency at which this phenomenon occurs is determined by the 'gyromagneltifc resonance in the ferromagnetic ceramic material, the gyromagnetic resonance being governed by the magnetisation of the said material and consequently by the applied static magnetic field.

A uni-directional waveguide attenuator owes its nonreciprocal property to the fact that it operates on an aspect of an electromagnetic field which depends on the direction of propagation, namely, a condition of circular polarization of the magnetic field vector. If a region of pure circular polarisation exists, a theoretically perfect operation can be obtained.

It is an object of the present invention to provide a unidirectional attenuator arrangement in a waveguide.

According to the present invention there is provided a unidirectional attenuator arrangement comprising a main waveguide along which electromagnetic energy is normally propagated in a mode approximating the T.E.M. mode, a waveguide stub coupled to the main waveguide for developing a circularly polarised magnetic field, ferromagnetic ceramic material located within said circularly polarised magnetic field, and means for applying a static magnetic field to condition said ceramic material for gyromagnetic resonance.

Example of waveguides along which electromagnetic energy is normally propagated in a mode approximating the T.E.M. mode are strip line waveguides and co-axial line waveguides.

For a better understanding of the invention and the method of carrying the same into effect, reference will now be made to the accompanying drawings in which:

Fig. 1 is a cross-sectional view of a unidirectional attenuator arrangement; and

Fig. 2 is a plan view of part of the arrangement shown in Fig. 1 along AA.

By a strip line Waveguide is meant a waveguide (transmission line) which includes strip-line conductors disposed in a spaced parallel relation, and in which electromagnetic energy is normally propagated in a mode approximating the T.E.M. mode. An example of a strip-line waveguide is a waveguide including two striplike conductors disposed in dielectrically spaced parallel relation a small fraction of a quarter wavelength apart with one of the strip conductors wider than the other ice to present thereto a planar conducting surface. Another example is a waveguide having three strip-like conductors disposed in dielectrically spaced parallel relation a small fraction of a quarter wavelength apart with two of the strips wider than the third, and on opposite sides thereof, such a strip-line being known as sandwich form. The dielectric materials may be polyethylene, polystyrene, fibreglass, or other suitable material of dielectric quality, or if the waveguide structure permits, the dielectric may be a gas such as air or nitrogen.

Fig. 1 is a cross-sectional view through a main stripline waveguide having a strip-like conductor 1, 1 on one surface of a dielectric 3 and having a ground plane 4 on the other surface of the dielectric. The dielectric 3 and the ground plane 4 are considerably wider than the strip-like conductor 1, 1' as shown in Fig. 2.

Electromagnetic energy is propagated along the stripline waveguide in a mode. approximating the T.E.M. mode, the magnetic vectors being transverse of the strip conductor 1, 1. A strip-line stub 5 (Fig. 2) extends at from, and is integral and co-planar with the strip-line waveguide. The strip-line stub 5 is a striplike conductor co-planar with the strip-like conductor 1, 1, the same dielectric 3 and the same ground plane 4 as the strip-line waveguide the width of the strip-like conductor 5 being preferably narrower than the conductor 1, 1'. When the stub 5 is of correct length in relation to the line impedances in the waveguide, theoretically, an odd number of A3 wavelengths, then the current at the stub base and the transmitted current in the main strip-line waveguide past the stub are substantially equal in magnitude and are in phase quadrature. Thus at the junction of the stub 5 and the strip-line waveguide the magnetic vectors are substantially equal in magnitude and are in phase quadrature, thereby resulting in a region of circular polarisation of the magnetic vector.

A piece of ferromagnetic ceramic material, such as a cylinder 6 of nickel-zinc ferrite having a low Curie point, is positioned within the region of the circularly polarised magnetic field, namely, between the strip-like conductor 1, 1 and the ground plane 4 as shown in Fig. l. The diameter of the cylindrical ferrite part 6 is 0.13 cm. and the length is equal to the thickness of the dielectric of the strip line. The ferrite part may conveniently be positioned by drilling a hole through the ground plane, di-

electric and strip-conductor, inserting the ferrite within the hole and then soldering the holes in the strip conductor and ground plane. The ferrite is located at the junction of the stub 5 with the main strip-line, although the exact location is not critical. The presence of the ferrite part 6 will probably modify the circularly polarised magnetic field and adjustment of the length of the stub 5 may be required to correct any modifications. An auxiliary strip-line stub 9 may be provided to improve the matching in the main strip-line waveguide. This auxiliary stub 9 is positioned symmetrically opposite to the stub 5 and utilises the dielectric 3 and the ground plane 4 as shown in Fig. 2. The stub 5 may, for example, be of a length equal to of the operating wavelength and the auxiliary stub 9 may be equal to A; of the operating wavelength.

A magnet is disposed with its pole-pieces 7 and 8 on opposite sides of the ferrite part 6, the pole-pieces 7 and 8 being north and south respectively. The magnetic field across the ferrite part 6 is of a definite sense according to the desired direction of attenuation, and of field strength to condition the ferrite for gyromagnetic resonance. In the present embodiment the field strength is equal to 500 oersteds, for a wave frequency of 4000 mc./s.

A wave at a particular frequency and travelling from the part 1 of the strip-like conductor 1, 1' to the other part 1 will be practically unattenuated as it passes the ferrite part 6, whereas the same wave, travelling in the reverse direction along the waveguide, will produce a substantially circularly polarised magnetic field of such sense that the wave will be substantially attenuated. A ratio in attenuation of 30 to 1 in the different directions has been obtained at the frequency 4000 mc./s.

While the principles of the invention have been described above in connection with a specific embodiment, it is to be understood that this description is made only by way of example and not as a limitation on the scope of the invention. In the case of a strip-line waveguide in the sandwich form, the ferrite is inserted, within the region of circular polarisation, on both sides of the central strip conductor. This may be effected in the manner previously described, namely by drilling a hole through the waveguide, inserting the ferrite part so as to be flush with the internal surfaces of the outer ground planes, and soldering the holes in these ground planes. If a co-axial line is used it will be appreciated that in the region of the stub, where the side arms in the outer casing extend from the main line, the field distribution is similar to that of the sandwich form strip line waveguide.

What we claim is:

l. A uni-directional attenuator arrangement comprising a main waveguide adapted for the propagation of energy therealong in a mode approximating the T.E.M. mode, a waveguide stub coupled in shunt with said main waveguide at an intermediate junction point whereby there is developed at said junction a transition region of circular polarization of the magnetic field, said main waveguide and said stub waveguide being disposed with their respective axes of wave propagation in mutually perpendicular relationship at said junction point, a ferrite located within said region of circular polarization, and means for applying a static magnetic field transverse to 4 the plane of said circular polarization to condition said ferrite for gyromagnetic resonance.

2. An arrangement according to claim 1, wherein the main waveguide is a strip-line waveguide and the waveguide stub is a strip-line stub, each strip-like conductor of the main strip-line waveguide being co-planar with and integral with each associated strip-like conductor of the strip-line stub.

3. An arrangement according to claim 1, further including an auxiliary stub positioned symmetrically opposite said waveguide stub so as to improve the matching characteristics of said attenuator.

4. A unidirectional attenuator arrangement as in claim 3, wherein said waveguide stub is of a length equal to three-eighths of the operating wavelength of said attenuator and the auxiliary stub is of a length equal to one-eighth of the operating wavelength of said attenuator.

5. An arrangement according to claim 1, wherein said ferrite is a nickel-zinc ferrite.

References Cited in the file of this patent UNITED STATES PATENTS 2,755,447 Engelmann July 17, 1956 2,767,379 Mumford Oct. 16, 1956 2,892,161 Clogston June 23, 1959 2,922,125 Suhl Jan. 19, 1960 FOREIGN PATENTS 163,740 Australia June 29, 1955 1,002,417 Germany Feb. 14, 1957 541,439 Italy Mar. 29, 1956 OTHER. REFERENCES Proceedings of the I.R.E., vol. 44, No. 8, August 1956, page 2a. 

